In general, an automatic transmission uses a planetary gear device as a multiple-stage speed change mechanism which changes the rotation force transferred from an input shaft and transfers changed rotation force to a driving shaft.
The planetary gear device is constructed such that an element serves as an input element and another element serves as a reaction element to transfer speed-changed rotation force to an output element.
In the planetary gear device, in order to allow respective elements to serve as the input element, the reaction element and the output element, a certain element of the planetary gear device is connected by a hydraulically acting friction element, and the hydraulically acting friction element is actuated or deactuated by application or release of a control pressure from a hydraulic control system.
The hydraulically acting friction element is constituted by a clutch which transfers the rotation power of an input shaft and a brake which secures one element of the planetary gear device.
If a hydraulic pressure is supplied to the clutch, a piston is moved and brings multiple clutch plates and multiple clutch disks into close contact.
Since the plates are spline-coupled to a retainer and the disks are spline-coupled to a hub, if the plates and the disks are brought into close contact, the power of the retainer is transferred to the hub.
A balance chamber and a return spring are arranged between the piston and the retainer. Since a lubricant is filled in the balance chamber, centrifugal force in a piston-acting direction by a working fluid may be offset.
Accordingly, if the pressure of the working fluid is released, the piston is returned to its original position by the returning force of the return spring.
Meanwhile, in a dual clutch device, two pistons are used, and a balance chamber and a return spring are provided for each of the two pistons.
The background art of the present disclosure is disclosed in Korean Unexamined Patent Publication No. 10-1996-031833 (published on Sep. 17, 1996 and entitled ‘Clutch Assembly Structure of Multiple Disk Clutch’).